Blocking TOR exit nodes?

[dzeanah]

Not sure how to do this.

I should start by saying that I'm a believer in TOR, and have been known to dedicate real amounts of bandwidth to the project on occasion.

However, I also run a reasonably busy forum and we are seeing more and more spammers appear from behind TOR exit nodes.

Is there a simple way to block these users at the firewall?  Or at least get a measurement of how many users access via TOR exit nodes so I can understand how many would be inconvenienced by taking this sort of action?

Thanks.  I'm sure I'm not the only one considering this.